IF it is only one tube doing it, then you'll need to retube at least that channel. Tubes are *microphonic* and the degree to which they do that varies quite a lot, as does their internal self noise - so that's one set of parameters which are used to select tubes out of a batch.
Keep the 300Bs... they're nice...
Check the drivers, as suggested, and swap channels for certain identification...
Putting anything on the outside of the tube won't do very much to eliminate the internal microphonics - of course the less you *excite* the resonance, the less of it you will hear.
Keep the 300Bs... they're nice...
Check the drivers, as suggested, and swap channels for certain identification...
Putting anything on the outside of the tube won't do very much to eliminate the internal microphonics - of course the less you *excite* the resonance, the less of it you will hear.